1. Field of the Invention
The present invention relates to a method for making a piezoelectric element, and more particularly, to a method for making a piezoelectric element using a gas deposition technique.
2. Description of the Related Art
Piezoelectric elements have the piezoelectric effect in which an electric field is generated by strain and the reverse piezoelectric effect in which strain is caused by the application of an electric field. As the piezoelectric material, lead zirconate titanate (hereinafter abbreviated as “PZT”) to which various trace metals, such as strontium and barium, are added is mainly used. Conventionally, piezoelectric elements are produced by a method including mixing of raw material powders, compression, sintering, machining, application of an electrode material, and polarizing treatment to impart piezoelectric properties.
With the recent miniaturization of devices, it is becoming necessary to mount piezoelectric elements on thinner, smaller spots. Since oxide-based piezoelectric materials, such as PZT, are brittle, there is a limit to a decrease in the thickness of the piezoelectric elements produced using such piezoelectric materials. It is difficult to decrease the thickness to about 0.1 mm or less. In the high-frequency bands, the loss from adhesives must also be taken into consideration. Therefore, in order to prepare piezoelectric films for producing thinner piezoelectric elements without including a bonding step, various film deposition methods, such as gas deposition techniques, hydrothermal synthesis, and sol-gel processes, have been invented, and piezoelectric elements using the piezoelectric films prepared by these methods have been manufactured by way of trial. Among these methods, in view of forming so-called “thick films” of several micrometers to several tens of micrometers, gas deposition techniques are receiving attention.
An apparatus for producing piezoelectric elements using a gas deposition technique includes at least an ultra-fine particle-floating chamber, a film-forming chamber, and a transport pipe. The gas deposition technique is a dry method for forming films, in which ultra-fine particles generated or prepared in the ultra-fine particle-floating chamber are introduced into the film-forming chamber through the transport pipe by a carrier gas, such as an inert gas, and are sprayed at a high speed from a nozzle to directly draw a pattern.
Ultra-fine particles may be, for example, generated in the ultra-fine particle-floating chamber by heating and vaporizing the raw materials in an inert gas using arc heating, resistance heating, or the like. Alternatively, ultra-fine particles of PZT or the like prepared in advance may be used. Ultra-fine particles with an average particle size of 0.1 to 1 μm are usually used. The ultra-fine particles are used, for example, in a dry method for forming films, in which a carrier gas containing the ultra-fine particles or the ultra-fine particles placed in the ultra-fine particle-floating chamber and floated by a carrier gas are introduced into the film-forming chamber through the transport pipe, for example, using a differential pressure between the pressure in the ultra-fine particle-floating chamber and the pressure in the film-forming chamber. By spraying from the nozzle at a high speed, a pattern is directly drawn on a substrate or the like.
The gas deposition technique is characterized in that the film deposition rate is higher and film deposition temperature is lower compared with sol-gel processes or hydrothermal synthesis. Since ultra-fine particles have smaller particle sizes than those of ordinary fine particles, they have very large specific surface areas and become highly active. The ultra-fine particles are bonded with each other by collision energy with the substrate, and thereby a piezoelectric film is formed. For example, Japanese Patent Laid-Open No. 6-285063 (patent document 1), No. 11-334066 (patent document 2), No. 11-330577 (patent document 3), and No. 2001-152360 (patent document 4) disclose methods for making piezoelectric elements, in which piezoelectric films are formed by gas deposition techniques, electrodes are applied to the films, and then polarizing treatment is performed.
A mechanism in which PZT exhibits piezoelectric properties will be briefly described below. In the crystal structure (perovskite structure) of PZT, there is a misalignment in the center of mass between the positively charged and negatively charged ions. The misalignment results in an electric dipole. PZT has spontaneous polarization and exhibits piezoelectric properties.
In the PZT grains, domains with sizes of 0.2 to 0.4 μm are formed, and each domain has a spontaneously polarized electric dipole and has piezoelectric properties. In the conventional process of forming a PZT piezoelectric film, a polycrystalline sintered compact is produced. Since the crystal axes of the individual crystal grains are randomly aligned, the spontaneous polarizations of the domains in the individual crystal grains are also randomly oriented. Therefore, the electric dipoles of the individual domains cancel each other out overall, and the piezoelectric properties of the sintered compact disappear. In order to impart piezoelectric properties to the sintered compact, it is necessary to carry out a step of aligning the orientations of the spontaneous polarizations of the individual domains (polarizing step).
Polarizing conditions in the polarizing step vary depending on the compositions of PZT. In general, an electric field of 1 to 5 kV/mm is applied for 30 minutes to 1 hour at 100° C. to 150° C. In the polarizing step, by applying the electric field under the conditions described above, the electric dipoles in the domains of the individual crystal grains are aligned in a certain direction. Since the sintered compact is polycrystalline, and the individual crystal grains physically stick to the adjacent crystal grains, the crystal grains are gradually deformed by applying heat over a certain period of time so that the orientations of the electric dipoles of the individual crystal grains are displaced.
Accordingly, in the methods for producing piezoelectric elements which include polarizing treatment, various steps, such as heating, voltage application, and cleaning, are required, and also, since strain is caused due to the alignment of the electric dipoles, dimensional accuracy is degraded.
Hydrothermal synthesis is known as a film deposition method without including polarizing treatment. However, due to the drawbacks, such as low deposition rate and residual solvent, it has been confirmed that hydrothermal synthesis is not suitable for forming piezoelectric thick films.